The subject matter of the invention is a surface wave transversal filter as used, in particular, as an intermediate frequency filter or precise-frequency transmission filter in consumer electronics.
Surface wave transversal filters (“SAW transversal filters” or “transversal filters”), are known, by way of example, from “SAW Components, Data Book 1996” from Siemens Matsushita Components and specifically in the section “General technical information” on pages 27 ff. These transversal filters are constructed on a crystalline piezoelectrical substrate and have two interdigital transducers which are used as input and output transducers.
The input transducer, to which the signal to be filtered is applied, has a weighting which is used to shape the transfer function. In contrast, the output transducer is short relative to the input transducer and is unweighted, and thus has the same overlap lengths for all the electrode fingers on the interdigital transducer. Such an unweighted interdigital transducer has a transfer function
            sin      ⁡              (        x        )              x    .
The exact form of the weighting of the input transducer is the result of optimization performed using inherently known suitable software tools. If the result of such optimization, i.e., a filter optimized by software, has a characteristic measured in real terms which does not match the desired characteristic, then various methods for measurement correction are known. Essentially, this is achieved by varying the overlap length of the electrode fingers in the input transducer. In a number of cases, however, such measurement correction does not result in the desired success following software optimization. In particular, such non-optimum filters can have a transfer response which shows an attenuation response for particular frequencies which is unsatisfactory. By way of example, such a transfer curve can have too little selection with respect to an adjacent channel or with respect to an interfering frequency, for example, the mirror or image frequency. If these conventional measurement corrections are not enough to produce the desired transfer characteristic, then redesign is necessary, i.e., reoptimization with altered constraints, which is complex to implement and also does not necessarily result in an improved transfer characteristic and, in particular, an improved trap characteristic (i.e., filtering a particular frequency).